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The Technology and Operation of Piezo Sounder
The Technology and Operation of Piezo Sounder
Basic Principles
Impedance Characteristics
Mounting
A piezoelectric ceramic element is
a sintered body of many crystals
(Poly-crystals).Distortion of this
Crystal occurs when a stress is
applied to the element, either
thermally,
mechanically
or
electrically.
These
distortions
create many possible uses
including alarm and sensor
applications.
The
equivalent
circuit
for
piezoelectric elements is shown in
Fig.3. The mechanical resonance
of the element is show by R, L, C
where L and C determine the
Resonant frequency (Fig.3).
(1) Node support
The sound element shown in Fig.2
(a) is node mounted, allowing it to
vibrate in a free state. The node, a
circumference where no vibration
takes place, is created as shown
by the broken line Fig. 1.
1
Fo=
Mounting at the node causes the
least mechanical suppression of
Because the shunt capacitor is
vibration, thus allowing the
larger than the series combination greatest amplitude. Hence this
In using piezoelectric elements in the total impedance is capacitive.
mounting method, as illustrated in
audible output an application, a
Figure 5(a), gives the highest
metal plate is attached to the
Modes of vibration and
sound pressure output and the
ceramic element because the
supporting
methods
for
the
most stable stable oscillation
resonant frequency of the ceramic
sound
element
frequency of the three choices. As
is too high to produce an audible
tone by itself. This metal plate Three principal modes of vibration a result, this is the most
vibrates as shown in Fig.1 due to can be created in the element appropriate design for high output,
the contraction and expansion of depending on the style of self-drive applications.
the Piezo ceramic, and an audible mounting. This is illustrated in
signal is produced.
Fig.2.
2π
L1C1
Fig.3.
Fig. 1.
Bending Vibration Node
Fig. 2.
Vibrating Mode of Piezo sounder
Fig. 4.
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The Technology and Operation of Piezo Sounder
(2) Edge support
Fig. 2(b) shows the mode of
vibration when the sound element
is supported at the edges. In this
mounting configuration, the whole
sound plate vibrates up and down
as is illustrated by the broken line
in the diagram. Hence, the edge
method as illustrated in fig.5(b),
suppresses
the
fundamental
resonant frequency by moving the
node. This offers the possibility of
a wide frequency response, and is
most advantageously used with
external drive.
(3) Center Support
Circuit design considerations 3. DC Precaution
In order to prevent depolarization
1. Driving Wave
The Piezo elements may be driven
by either sinusoidal, pulsed, or
square wave, depending upon the
particular application. If a sine
wave is used, the device will
operate at a frequency lower than
the resonant frequency (Fo) with a
lower sound pressure level. The
reason for this is the loss of energy,
through the time lag between peak
Deflections as shown in Fig. 7. It is
important that a clean sinusoidal
signal be provided, as any clipping
of the waveform can result in
frequency instability. If square
waves or pulsed waves are used
to drive the elements, a higher
acoustic output will be realized,
along with an increase in harmonic
levels. A parallel capacitor can
reduce these harmonics.
Fig.2(c) shows the mode of
vibration when the sound element
is supported at the center. As the
main vibration area is forcefully
supported, large sound pressure
levels are not possible when this
method is used. This too is
appropriate for external drive but 2. Driving Frequency
due to design difficulties center For maximum output, a frequency
support is not useful as and alarm. of between 500Hz and 4KHz
should be used, as recommended
by the specific part chosen.
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of the ceramic elements it is
necessary that every precaution
be taken to prevent them from
being subjected to direct current.
4. High voltage Precautions
Voltage
higher
than
those
recommended by specification
can damaged the ceramic, even if
applied for short durations. Due to
the strength of the piezoelectric
effect, high voltage can cause the
crystals to break the sintered
bonds, resulting impermanent
damage.
Significantly
higher
sound pressure levels will not be
achieved by voltages higher than
those
recommended
by
specification.
The Technology and Operation of Piezo Sounder
5.Booster coil Applications： element and encased air can be 10.Soldering Recommendations
When using a booster coil, do not matched. This case can be
exceed
voltage designed using the following
recommendations ,as the coil will （Helmholtz’s equation）
heat up, passing too much current
to the transistor.
C
Fo=
4a2
2π
6.Shock：
d2h( t+ ka )
Mechanical impact on buzzers or
fo=Resonant frequency of Cavity (Hz)
elements can generate high
c=Sound velocity 34.4*103
voltages that can seriously harm
cm/sec@24℃
drive circuitry. Suitable diode
protection is advisable in
a=Radius of sound emitting hole (cm)
applications where mechanical
d=Diameter of support
shock is possible. Zener diode
h=height of cavity (cm)
shown as Figure 7a; Schottky
t=Thickness of cavity
diode shown as Figure 7b.
k=Constant ≒1.3
7.Mounting Glue：
9.Electrostatic Capacitance
It is necessary to match the output
impedance of the oscillator with
the transducer impedance in order
to get maximum sound pressure
8.Design of resonating Case： level from the transducer. The
actual electrostatic capacitance
When an element is supported
can be calculated from the
and has no case, the sound
following formula.
pressure level is small. This is
because the acoustical impedance
132.064D2
of the elements does not match
C=
that of any open air loading.
t
pF
However, by building a resonating
D=diameter
of
electrode (cm)
Case, the acoustical impedance of
t=Thickness
of
ceramic (cm)
the acoustical impedance of the
Proper application of mounting
glue is necessary to produce
adequate sound pressure levels.
The desired location for soldering
lead wires on an element is the
point nearest to edge of the silver
surface. The desired location for
soldering a lead to the metal plate
is the area between the end of the
plate and the end of the ceramic.
Below are the conditions for
soldering.
Ceramic
Metal plate
(AG)
Soldering
Iron
Temp.
Time
Solder
Fig.7(a).
Fig. 6.
Fig. 7(b).
Fig. 8
(a) Edge Mount
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.
(b) Node Mount
25W
25W
330℃±30℃
0.5sec
Max.
2～4 sec.
As solder